Field of the Invention
The present invention relates to an organic light emitting diode display and a method of manufacturing the same. More particularly, the present invention relates to an organic light emitting diode display with improved ambient contrast ratio (ACR).
Discussion of the Related Art
Recently, various flat panel displays that are less bulky and lighter than cathode ray tubes (CRTs) are being developed. Examples of the flat panel displays include liquid crystal displays (LCDs), field emission displays (FEDs), plasma display panels (PDPs), electroluminescence devices (ELs), etc.
Organic electroluminescence devices are a self-luminous device, which uses an organic light emitting diode, and offer various advantages including fast response time, high luminous efficiency, high luminance, and wide viewing angle.
FIG. 1 is a diagram illustrating a structure of an organic light emitting diode. The organic light emitting diode comprises an organic electroluminescence compound layer that emits light, and a cathode and anode facing each other, with the organic electroluminescence compound layer sandwiched between them. The organic electroluminescence compound layer typically comprises a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, and an electron injection layer EIL.
An exciton is formed through an excitation when a hole and an electron injected into the anode and the cathode recombine at the emission layer EML, and the energy of the exciton causes the organic light emitting diode to emit light. An organic light emitting diode display displays an image by electrically controlling the amount of light generated from the emission layer EML of the organic light emitting diode illustrated in FIG. 1.
Organic light emitting diode displays (OLEDDs), which are electroluminescence devices using the characteristics of the organic light emitting diode, can be classified into passive matrix-type organic light emitting diode displays (PMOLEDs), and active matrix-type organic light emitting diode displays (AMOLEDs).
AMOLEDs display an image by controlling the current flowing through the organic light emitting diode using thin film transistors (hereinafter, “TFTs”).
FIG. 2 is an equivalent circuit diagram of a structure of a sub-pixel in an active matrix-type organic light emitting diode display (AMOLED). FIG. 3 is a top plan view illustrating a structure of a sub-pixel in the AMOLED. FIG. 4 is a cross-sectional view of the structure of the AMOLED taken along line I-I′ of FIG. 3.
Referring to FIGS. 2 through 4, the AMOLED comprises switching TFTs ST, driving TFTs DT connected to the switching TFTs, and organic light emitting diodes OLED connected to the driving TFTs DT. The TFTs of FIG. 4 are bottom-gate TFTs, but they are not limited thereto and may include other types of TFTs such as top-gate TFTs.
A switching TFT ST is formed at an intersection of a scan line SL and a data line DL. The switching TFT functions to select a sub-pixel. The switching TFT ST comprises a gate electrode SG branched from the scan line SL, a semiconductor layer SA, a source electrode SS, and a drain electrode SD. A driving TFT DT functions to drive the organic light emitting diode OLED of the sub-pixel selected by the switching TFT ST. The driving TFT DT comprises a gate electrode DG connected to the drain electrode SD of the switching TFT ST, a semiconductor layer DA, a source electrode DS connected to a power supply voltage source VDD, and a drain electrode DD. The drain electrode DD of the driving TFT DT is connected to the anode ANO of the organic light emitting diode OLED.
An organic light emitting layer OL is interposed between the anode ANO and the cathode CAT. The cathode CAT is connected to a ground voltage source VSS. An auxiliary storage capacitor Cst is disposed between the gate electrode DG of the driving TFT DT and the power supply voltage source VDD or between the gate electrode DG of the driving TFT DT and the drain electrode DD of the driving TFT DT.
The gate electrodes SG and DG of the switching TFT ST and driving TFT DT are formed on a substrate SUB of the AMOLED. A gate insulating film GI covers the gate electrodes SG and DG. The semiconductor layers SA and DA are formed on part of the gate insulating film GI overlapping the gate electrodes SG and DG. The source electrodes SS and DS and the drain electrodes SD and DD are formed on the semiconductor layers SA and DA, facing each other at a predetermined distance. The drain electrode SD of the switching TFT ST is in contact with the gate electrode DG of the driving TFT DT via a contact hole formed in the gate insulating film GI. The switching TFT ST and driving TFT DT with these structures are covered with a passivation film PAS.
Since many structures are formed on the substrate including the TFTs ST and DT, the surface of the substrate is not flat and has many step portions. The organic light emitting layer OL is beneficially formed on a flat surface to emit light uniformly and evenly. Accordingly, an overcoating layer OC is typically coated over an entire substrate for the purpose of flattening the surface of the substrate.
The anode ANO of the organic light emitting diode OLED is formed on the overcoating layer OC. Here, the anode ANO is connected to the drain electrode DD of the driving TFT DT via a contact hole formed in the overcoating layer OC and the passivation film PAS.
To define a sub-pixel area on the substrate where the anode ANO is formed, a bank BN is formed above the area in which the switching TFT ST, the driving TFT DT, and various wiring lines DL, SL, and VDL are formed. The anode ANO, exposed by the bank BN, becomes a luminous area. The organic light emitting layer OL is formed on the anode ANO exposed by the bank BN. The cathode CAT is formed on the organic light emitting layer OL.
The cathode CAT is formed in such a way as to cover the organic light emitting layer OL and the bank BN, and is deposited along a tapered surface of the bank BN. In order for the cathode CAT to have a good step coverage, an inflection part INF is formed along a curve of the tapered bank BN. The inflection part INF is a part which has a curved shape and is gently stepped.
The AMOLED may be used indoors as well as outdoors. In environments affected by external light sources, ambient contrast ratio is an important factor for the productivity and reliability of the AMOLED. When the AMOLED is used outside, an ambient light 2 from an external light source 1 with an extremely high brightness, such as sunlight, may enter the AMOLED and be reflected by the cathode CAT. The reflected light is then mixed with a self-luminous light 5 generated from the organic light emitting layer OL, which may prevent the user from properly perceiving an image presented by the AMOLED. That is, the AMOLED may have a very low ambient contrast ratio (ACR) depending on the intensity of the ambient light 2 generated from the external light source 1.
Notably, a diffused reflection, but not a specular reflection, may occur when the ambient light 2 from the external light source 1 enters the AMOLED. Diffusely-reflected and scattered rays 4 disturb the self-luminous light 5 generated from the organic light emitting layer OL. The diffuse reflection that disturbs the self-luminous light 5 is mainly due to the inflection part INF at the second electrode CAT formed along the tapered surface of the bank BN.
For instance, FIG. 5 shows a range of diffuse reflection of an ambient light with an incident angle θ1 (see FIG. 4) of 45° off the inflection part INF (see FIG. 4), in the conventional AMOLED. It can be seen that most of the ambient light is reflected at angles θ2 (see FIG. 4) between 40° and 60°, but some scattered rays having angles of reflection θ2 between 10° and 60° exist. Especially, a substantial amount of scattered rays having an angle of reflection θ2 in a range of approximately 30° exists.
In a case where a navigation system is installed in the middle of a vehicle's interior and the user is sitting in the left or right seat, such a scattered light in a range of approximately 30° is within the user's field of vision. Thus, an ambient light coming from the sides at 45° is diffusely reflected within the user's field of vision. The diffusely-reflected and scattered light further decreases the ambient contrast ratio of the AMOLED, thus making it difficult to produce vivid images and resulting in low productivity and reliability.
Typically, a polarizer, antireflective films, etc. with low reflectivity and high transmittance have been used as one of the solutions to the above-described problem. Although the polarizer, antireflective films, etc. can improve ambient contrast ratio, they may reduce the transmittance of the self-luminous light emitted by the organic light emitting layer, thereby leading to a decrease in the luminance of the display device and higher power consumption. Moreover, the application of a polarizer with low reflectivity may increase manufacturing costs and require an additional process with an additional processing time.